Conventionally, available as a tapping screw used for screwing directly into a metal plate without making a screw-hole is a tapping screw having a conical portion at the end thereof with a tip angle of 40 deg to 50 deg and formed of single-start threads roughly pitched, which is specified in JIS standards and commonly known.
However, the conventional tapping screw is only applicable for a thin steel sheet of less than 1.0 mm in thickness because of having no drill portion as in a drill screw and requiring a high torque for screwing the conical portion.
Accordingly, in order to enable screwing into a metal plate as thick as possible without making a screw-hole, various improvements are proposed such as those having double-start thread or triple-start thread, changed in thread shape, provided with protrusion to make the conical tip portion capable of cutting, and provided with a cutting edge (for example, refer to Patent Documents 1 to 4).
However, all of tapping screws of this type so far proposed are still only applicable for a thin steel sheet of less than 1.0 mm in thickness, and cannot be used for fastening a steel plate of over 1.0 mm in thickness or two steel sheets of 0.8 mm max. in thickness because of greater screwing resistance. Also, in the case of the one provided with a protrusion or cutting edge at the conical tip portion to make it capable of cutting, it is difficult to form by using a reversing device and there arises a problem in making it practically usable. Therefore, in case of fastening a steel plate of over 1.0 mm in thickness or two steel sheets of 0.8 mm max. in thickness, a drill screw having a drill portion at the screw end thereof is usually employed.
When securing a collar steel plate of roof material by using drill screws or fixing a draining portion, gutter, windbreak, signboard or the like on a wall steel plate, helical chips generated due to the cutting action of the drill screw remain without being carried away by the wind and rain, causing red rust to be generated, and as a result, it is sometimes required to remove them at the time of construction. Also, chips falling down in the building may affect the indoor environment depending upon the using purpose of the building. Accordingly, the development of technology that enables fastening by screwing without screw-hole causing no generation chips is expected.
Further, in an assembly line of the automobile industry, drill screws were tried, but it was rather avoided because helical chips generated due to the cutting action were scattered to electric wiring connections and others, causing damages to the electric system. Accordingly, if screwing without screw-hole causing no generation of chips is realized, it will bring about a new development in the automobile industry.